Corrugated paper board or liner board is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls forming a substantially sinusoidal or serpentine cross-section in the sheet. An adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes and a non-corrugated or planar cellulosic liner is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced element. The single-faced element may be used as is in certain applications as a liner or buffer material within a container. More commonly adhesive is applied to the flute tips of the single-faced element and a second liner sheet is subsequently applied to the adhesive liner in a "double facer" operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In a so-called "hot-plate" dryer section a continuously moving belt with a hot-plate or platen surface is used to partially remove water from the adhesive bond line and establish a cured adhesive bond at temperatures of about 300.degree.-400.degree. F.
The adhesives which have been developed to form the corrugated board or paper board are generally starch base adhesives. The earliest corrugating board adhesive patent, U.S. Pat. No. 2,102,937, discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, alkali and borax in a water base. This adhesive has formed the basis for the manufacture of essentially all corrugated paper board to date. This adhesive is a carrier adhesive, wherein a portion of the starch forms a carrier gelatinized phase and the balance of the starch forms an ungelatinized phase. Under conditions of heat and pressure the ungelatinized starch rapidly gelatinizes, absorbs water and causes a rapid increase in viscosity and adhesivily of the starch-adhesive composition. Borax increases the level of viscosity as well during final gelatinization.
More recently a number of patents have disclosed a corrugating board adhesive that can be characterized as a conventional starch-based adhesive containing finely divided solid particles of a substantially room temperature insoluble polyvinyl alcohol. The water insoluble polyvinyl alcohol disclosed in these patents becomes soluble at an elevated temperature. A substantially insoluble polyvinyl alcohol is used in these patents since the common belief of the corrugating industry is that soluble polyvinyl alcohol will gel in the presence of borax containing starch based adhesives. The use of insoluble polyvinyl alcohol permits the use of the polyvinyl alcohol in starch based adhesives since at storage or ambient temperatures the polyvinyl alcohol, in solid form, cannot interact with starch or borax. After application to the flute tips, at conditions of high temperature and pressure, the polyvinyl alcohol can rapidly dissolve and substantially increase the viscosity and bond strength of the bond line between the corrugating medium and the liner material.
The speed at which corrugating machines can be run is limited by the rapidity of viscosity increase in the bond line between the liner and corrugated medium. As the viscosity of the adhesive rises under conditions of heat and pressure in the corrugating machine, a "green bond" is formed which holds the components together until the adhesive system forms the final heat-cured corrugated bond. The green bond relies solely on the viscosity of the product to maintain the integrity of the product during curing.
Conventional starch based adhesives rely on the response of borax and ungelatinized starch under conditions of heat and pressure to increase the viscosity of the adhesive during green bond formation. More recent corrugating adhesives containing the additionally insoluble polyvinyl alcohol utilize the increase in viscosity that results as the polyvinyl alcohol is solubilized at high temperature. However, both the conventional and the solid polyvinyl alcohol containing conventional adhesives increase viscosity at a rate that leaves substantial room for improvement. If the rate at which viscosity increase occurs in the bond line can be increased, then the corrugating machines can be run at a substantial increase in speed resulting in an increase in productivity.
Accordingly a substantial need exists for a corrugating adhesive having a faster set or a substantial increase in the rate of viscosity development or in the rate of green bond formation during the manufacture of corrugated board.